1. Field of the Invention
This invention relates to computer graphic systems and particularly to computer graphic systems providing color television raster scan video signals as an output.
2. Discussion of the Prior Art
Raster scan video graphic systems have an advantage over vector display types of graphic systems in that they can produce much more complex graphic images which include multicolor areas, shading and blending. However, to achieve this flexibility a raster scan system must process a huge number of picture elements or pixels corresponding to individually sampled points within a row and column pixel matrix for each frame of a visual image. In such a system a frame store is typically provided to store a byte of information for each pixel of a video image. Such frame stores are extremely large and expensive. In some systems the three color components of a visual image are stored within the single byte of storage capacity for each pixel location. This results in an unsatisfactory video image with poor definition of color and intensity throughout the image.
In other systems known as color map systems a second, much smaller color map memory is provided which is addressed by the one byte of video information at each pixel location of the frame store memory. The color map memory has a typical configuration of 256 words.times.24 bits. The 24 bits permit one 8 bit byte of resolution for each of three primary components of a color video signal. The artist is then able to store one of 256 colors at each color map address location and then store a byte of data identifying one of these color defining locations at each pixel location within the frame store. Such an arrangement is capable of producing a high quality video image but is limited in that it is limited to 256 preselected colors out of a combination of millions of possible colors which can be distinguished on a television video display. The limitation of 256 predefined colors is not sufficient to permit general low pass filtering of television quality complex pictures.
Various examples of raster scan data graphic systems are disclosed by the following U.S. Pat. Nos.: 4,093,996; 4,117,473; 4,129,859; 4,156,914; 4,158,838; 4,189,743; and 4,189,744.